Charging System

The Pelletron® charging chain used in NEC
accelerators was developed in the mid 1960s as an improvement over the older
Van de Graff charging belts. These belts suffered from a number of operational
difficulties including terminal voltage instability and susceptibility to spark damage.
Also, they generated belt dust which necessitated frequent cleaning inside the accelerator
tank. The chain rapidly proved to be more durable than the old belts, while producing a greater
terminal stability than had been possible before. It
eliminated the belt dust problem as well. The chain does not limit ultimate terminal potential,
and it is in use in electrostatic accelerators up to and above 25 MV.

In addition to being far superior to the belt-driven Van de Graaff, Pelletron
charging systems also offer significant advantages over solid-state charging systems.
These systems have fragile electronics in the high-voltage column that are
prone to problems which are both difficult to diagnose and expensive to repair. Because
they are susceptible to spark damage, solid-state systems require long
times to condition up to rated voltage.

Pelletron chains are made of metal
pellets connected by insulating nylon links and are charged by an induction scheme that
does not use rubbing contacts or corona discharges. For a positive terminal
Pelletron, the negatively-charged inductor electrode pushes electrons off the
pellets while they are in contact with the grounded drive pulley. Since the pellets
are still inside the inductor field as they leave the pulley, they retain a net positive
charge. The chain then transports this charge to the high-voltage terminal, where the
reverse process occurs. When it reaches the terminal, the chain passes through a
negatively-biased suppressor electrode which prevents arcing as the pellets make contact
with the terminal pulley. As the pellets leave the suppressor, charge flows smoothly onto
the terminal pulley, giving the terminal a net positive charge. Most Pelletrons,
as shown in the charging system diagram, employ "down-charging" as well as "up-charging."
Down-charging works identically to up-charging, except the inductor/suppressor polarities
are reversed, and it effectively
doubles the charging current capacity of the chain. For the configuration shown here,
small, slightly-conductive "pickoff pulleys" provide the voltages for the terminal
suppressor/inductor electrodes by drawing a tiny amount of charge from the chain, each pulley
thus biasing the opposing electrode. This technique allows down-charging "for free," i.e.
without the use of HV supplies in the terminal. Depending on the particular design
options, the system delivers charging currents of 100 - 200 µA or more per
chain to the high voltage terminal. The drive pulleys, typically 30 cm to 60 cm in diameter,
and motors are supported on movable platforms which are counterweighted, automatically providing
proper chain tension.

There are well over 300 Pelletron chain assemblies in use in 24 countries as
part of the world's most advanced research systems. About 20 of these assemblies
are pellet chain conversions for the old HVEC Models ESTU, MP, FN and EN.